Earthquake Hazard Uncertainties Improved Using Precariously Balanced Rocks

Abstract

Probabilistic seismic hazard analysis (PSHA) is the state‐of‐the‐art method to estimate ground motions exceeded by large, infrequent, and potentially damaging earthquakes; however, a fundamental problem is the lack of an accepted method for both quantitatively validating and refining the hazard estimates using empirical geological data. In this study, to reduce uncertainties in such hazard estimates, we present a new method that uses empirical data from precariously balanced rocks (PBRs) in coastal Central California. We calculate the probability of toppling of each PBR at defined ground‐motion levels and determine the age at which the PBRs obtained their current fragile geometries using a novel implementation of cosmogenic 10 Be exposure dating. By eliminating the PSHA estimates inconsistent with at least a 5% probability of PBR survival, the mean ground‐motion estimate corresponding to the hazard level of 10 −4  yr −1 (10,000 yr mean return period) is significantly reduced by 27%, and the range of estimated 5th–95th fractile ground motions is reduced by 49%. Such significant reductions in uncertainties make it possible to more reliably assess the safety and security of critical infrastructure in earthquake‐prone regions worldwide. Rare earthquakes can be extremely destructive and costly, but existing hazard estimates for rare earthquake shaking are highly uncertain because observations are limited to historical records. This study utilizes unique prehistoric shaking constraints provided by precariously balanced rocks. At a site in Central California, we characterize the probability of toppling of such precariously balanced rocks and determine their age of formation. The precariously balanced rock constraints are used to directly eliminate estimates in the hazard model that are inconsistent with the preservation and antiquity of the rocks. These results dramatically improve the hazard model and significantly reduce uncertainties in the estimates. Our study demonstrates how constraints on seismic shaking for the geological past can be used to improve earthquake hazard estimates for the future. Precariously balanced rocks provide empirical data that can both validate and improve uncertain earthquake hazard models at long timescales This study greatly reduces the uncertainty (90% confidence interval) of earthquake hazard estimates by 49% Our new method is applicable at suitable sites globally where critical infrastructure or human populations coexist with earthquake hazards